Method and apparatus for reduction of fatigue and gust loads on wind turbine blades
An adjustable lift regulating device (30, 32, 40, 50, 52, 56, 60, 68, 72, 76) on an inboard portion of a wind turbine blade (28). The lift regulating device is activated to reduce lift on the inboard portion of the blade by causing flow separation (41) on the suction side (22) of the blade. To compensate for the lost lift, the blade pitch is increased to a running pitch that facilitates stalling on the outer portion of the blade in gusts. This provides passive reduction of fatigue and extreme loads from gusts while allowing full power production under non-gust conditions.
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This application is a continuation-in-part of U.S. patent application Ser. No. 14/164,879 filed on 27 Jan. 2014 and international patent application number PCT/US2013/064060 filed on 9 Oct. 2013, both of which are incorporated herein by reference.
FIELD OF THE INVENTIONThis invention relates generally to the field of wind turbines, and more specifically to methods and apparatus for reducing cyclic fatigue and extreme loads on wind turbine blades.
BACKGROUND OF THE INVENTIONWind turbines can encounter excess wind speeds beyond their structural design capacity or beyond what is needed for maximum rated power output. The “rated wind speed” of a wind turbine is the lowest wind speed at which it produces power at its rated capacity. Damaging blade loads can be sustained above this rated wind speed in what is called the ‘post-rated’ regime, typically 12-25 m/s or 15-20 m/s for example, depending on the turbine. To maximize efficiency before reaching rated power and to regulate power to a fixed level above rated wind speed, the turbine varies the blade pitch depending on torque/power output. Beyond the safe operating wind speed, the blades are feathered, and the rotor may be stopped or idled.
To maintain power production in the post-rated wind speed regime, the blade is pitched progressively towards feather with increasing wind speed. As a result, the outboard parts of the blade may have a near zero or negative angle of attack, at which they are at or near idle or are in a negative lift or “braking” state. In such conditions, most or all of the rotor torque is generated by inboard parts of the blade (
When operating at rated wind speed or less, the blade pitch is set for an angle of attack that maximizes power production and is associated with lift being generated at all blade radial positions. A sudden gust increases the angle of attack to a stall condition that reduces lift temporarily, thus protecting the blade from lift overloads and rapid load changes. However in the post-rated wind configuration with zero or negative angle of attack on parts of the blade, this natural load alleviation mechanism is reduced because the outboard blade sections are operating far from stall. In this configuration, a gust initially increases the angle of attack toward maximum lift, causing a large and rapid load increase on the outer portion of the blade, leading to higher amplitude fatigue load cycles.
The invention is explained in the following description in view of the drawings that show:
The inventors devised a method for reducing fatigue and extreme loads on a wind turbine in strong gusty winds by 1) increasing airflow separation on an inboard part of the blades, thus reducing lift during a steady state airflow; 2) increasing blade, pitch to restore the reduced lift during the steady state airflow, thus moving the angle of attack closer to the point of maximum lift and enhancing a stall response to gusts on the outboard portion of the blades. This allows the outboard blade portion to safely provide power in high winds, because it stalls in gusts that increase the angle of attack beyond its stall angle, thus unloading it. The angle of attack of an operating wind turbine blade is a function of the pitch angle of the blade, the rotational speed of the turbine, and the wind speed. When the wind speed increases as a result of a gust of wind, the angle of attack increases. A lift regulating device (or lift regulator) other than pitch is installed on the inboard blade portion, and is activated to reduce lift responsive to wind condition indicators such as wind speed and variability, blade pitch, and blade loading. After or during activating the lift regulator, the blade pitch is increased to restore the lift, thus increasing the angle of attack, and making the stall response more rapid because the blade is operating closer to the stall point on the outboard sections than it would operate without activation of the lift regulator. A gust that increases the angle of attack is more quickly unloaded because stall occurs sooner than in the prior art position. This reduces fatigue and extreme loads, especially when operating beyond rated wind speed. The method is enabled by various embodiments of the lift regulation device herein. Any embodiment may be activated in proportion to the wind condition. The lift regulating device may be a slow-acting device, thus minimizing activation stress and actuator power requirements. Alternately, activation of the lift regulating device may be delayed so as to take at least 5 or 10 times longer than the pitch response to avoid feedback between the two. Alternately, activation may be delayed by at least 5 or more seconds.
An inboard lift regulator may be embodied as a suction side spoiler 40 that causes flow separation 41 when deployed. The spoiler may be a hinged plate as shown, and may be actuated by known means such as hydraulics or electric motors. Flow separation reduces 41 lift.
The method and apparatus herein enable longer blade designs by reducing cyclic fatigue and extreme loads, which may enable larger rotors to be placed on the same nacelle/drivetrain. Longer blades provide higher annual energy production. The invention also reduces wear on the blade pitch system by eliminating the need for continuous fast pitch responses in high winds. The increased angle of attack causes local stalling that unloads each gust without the need for a fast mechanical response.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Claims
1. A method for operating a wind turbine, comprising:
- providing an adjustable lift regulating device other than pitch on an inboard portion of a wind turbine blade;
- reducing lift on the inboard portion of the blade at a first pitch of the blade by activating the lift regulating device responsive to a wind condition;
- increasing the pitch of the blade to a second pitch greater than the first pitch to regain the reduced lift, wherein the second pitch facilitates stalling of an outer portion of the blade in a gust that increases an angle of attack thereof;
- wherein the adjustable lift regulating device comprises a leading edge shell comprising a bottom edge hinged to the blade below a leading edge of the blade; and
- the activating comprises pivoting a top edge of the lift regulating device forward and away from the blade to a position forward and above the leading edge of the blade.
2. The method of claim 1, further comprising determining the wind condition as a function of wind speed and wind speed variability.
3. The method of claim 2, further comprising delaying the activating for a time period greater than a short period variability of the wind speed.
4. The method of claim 1, wherein the first pitch is a first post rated pitch, and further comprising reducing the pitch of the blade from a running pitch to the first post rated pitch responsive to the wind condition, and activating the lift regulating device responsive to reaching the first post rated pitch.
5. An apparatus for a wind turbine, comprising
- a control unit that determines a wind condition; and
- an adjustable lift regulating device other than pitch on an inboard portion of a wind turbine blade, wherein an activation of the lift regulating device reduces a lift coefficient on the inboard portion of the blade;
- wherein the control unit is operable to activate the lift regulating device responsive to the wind condition at a first pitch of the blade, and then increases the pitch of the blade to a second pitch greater than the first pitch, wherein the second pitch facilitates stalling of an outer portion of the blade in a gust that increases an angle of attack thereof;
- wherein the adjustable lift regulating device comprises a leading edge shell comprising:
- a bottom edge hinged to the blade below a leading edge of the blade; and
- a top edge that controllably pivots forward and away from the blade to a position forward and above the leading edge of the blade.
6. A wind turbine comprising the apparatus of claim 5.
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Type: Grant
Filed: Dec 5, 2014
Date of Patent: Jun 27, 2017
Patent Publication Number: 20150098820
Assignee: SIEMENS AKTIENGESELLSCHAFT (München)
Inventors: Kristian R. Dixon (Boulder, CO), Edward A. Mayda (Thornton, CO)
Primary Examiner: Kenneth Bomberg
Assistant Examiner: Jesse Prager
Application Number: 14/561,309
International Classification: F03D 7/02 (20060101); F03D 1/06 (20060101);